1. Field of the Invention:
The present invention relates to a variable capacitor, and particularly to a variable capacitor in which the effective overlapping area between a stator electrode and a rotor electrode is varied through rotation of the rotor electrode relative to the stator electrode to thereby vary capacitance.
2. Description of the Related Art:
One type of variable capacitor is described in Japanese Patent Application No. 9-126586 filed on May 16, 1997 by the applicant of the present invention. FIG. 7 shows a variable capacitor 1 proposed in the application for patent.
Referring to FIG. 7, the variable capacitor 1 is primarily composed of a stator 2, a rotor 3, and a cover 4. A major portion of the stator 2 is formed of a dielectric, such as ceramic. The rotor 3 is formed of a metal, such as brass. The cover 4 is formed of a metal, such as stainless steel or copper alloy.
The above-mentioned elements of the variable capacitor 1 will next be described in detail.
The stator 2 generally has a symmetrical structure. Stator electrodes 5 and 6 are formed side by side in the stator 2. Stator terminals 7 and 8 are formed of a conductive film on the outer surfaces of corresponding end portions of the stator 2 so as to establish electric connection with the stator electrodes 5 and 6, respectively.
A dielectric layer 9 covering the stator electrodes 5 and 6 is formed of a portion of the dielectric that constitutes the stator 2.
As described above, the two stator electrodes 5 and 6 and the two stator terminals 7 and 8 are formed so as to impart a symmetrical structure to the stator 2, so that the orientation of the stator 2 need not be a consideration in the assembly of the variable capacitor 1.
The rotor 3 is placed on the stator 2 so that the rotor 3 comes in contact with the outer surface of the dielectric layer 9. A substantially semicircular rotor electrode 11 projects from the bottom side (as viewed in FIG. 7) of the rotor 3 so as to face the stator electrode 5 (and electrode 6) with the dielectric layer 9 disposed therebetween. FIG. 8 shows a bottom view of the rotor 3.
A protrusion 12 extending out as far as the rotor electrode 11 is also formed on the bottom side of the rotor 3 in a region other than that where the rotor electrode 11 is formed. The protrusion 12 serves to prevent an inclination of the rotor 3 which would otherwise result due to the presence of the rotor electrode 11.
A driver groove 13, which assumes a form of, for example, a square through-hole, is formed in the rotor 3 in order to receive a driver or a like tool used for rotating the rotor 3.
The cover 4 is attached onto the stator 2 while accommodating the rotor 3. The cover 4 allows the rotor 3 to rotate relative to the stator 2. The cover 4 has an adjustment hole 14 formed therein that allows the driver groove 13 to be exposed therethrough. Thus, when the rotor 3 is to be rotated, a driver or a like tool can be inserted into the driver groove 13 through the adjustment hole 14.
The cover 4 has a spring-action portion 15 formed around the adjustment hole 14. The spring-action portion 15 is partially in contact with the upper surface (as viewed in FIG. 7) of the rotor 3 to thereby affect a spring force which presses the rotor 3 against the stator 2. The spring-action portion 15 is formed in such a manner as to incline downward toward the center of the adjustment hole 14, thereby applying a spring force by means of a metallic material present around the adjustment hole 14.
A plurality of protrusions 16 are formed on the spring-action portion 15 at equal intervals along a rotational direction of the rotor 3. These protrusions 16 substantially come into point contact with the rotor 3. These protrusions 16 can be formed through, for example, embossing a metallic plate which constitutes the cover 4.
The cover 4 also has a rotor terminal 17 extending downward (as viewed in FIG. 7).
The variable capacitor 1 including the above-mentioned stator 2, rotor 3, and cover 4 is assembled in the following manner.
The rotor 3 is placed on the stator 2, and then the cover 4 is placed on the stator 2 in such a manner as to cover the rotor 3. Next, the cover 4 is attached onto the stator 2 while being pressed toward the stator 2 so as to press the rotor 3 against the stator 2.
In this case, the rotor terminal 17 integrated with the cover 4 is positioned so as to face the stator terminal 8 provided on the stator 2. In the structure illustrated in FIG. 7, the stator terminal 8 does not function as a stator terminal, and thus no electrical problem will arise.
In the thus-assembled state, the rotor electrode 11 faces the stator electrode 5 with the dielectric layer 9 disposed therebetween to thereby develop capacitance. In order to vary the capacitance through varying the effective overlapping area between the rotor electrode 11 and the stator electrode 5, the rotor 3 is rotated. The capacitance is externally presented between the stator terminal 7 and the rotor terminal 17. The stator terminal 7 is electrically connected to the stator electrode 5. The rotor terminal 17 is integrated with the cover 4, which is in contact with the rotor 3 on which the rotor electrode 11 is formed.
In the variable capacitor 1, the protrusions 16 formed on the spring-action portion 15 of the cover 4 are substantially in point contact with the rotor 3. Accordingly, the positions where the protrusions 16 press against the rotor 3 are reliably fixed. Even when the parallelism of the rotor 3 between the rotor-electrode side and the opposite side is poor or when the flatness of the rotor-electrode side or the opposite side of the rotor 3 or the flatness of a tip portion of the spring-action portion 15 is poor, a contact pressure can be applied in a stable manner to the rotor 3. That is, the above-described variations in machining are effectively "absorbed" in that they do not have an appreciable impact.
Thus, the rotor 3 is uniformly pressed against the stator 2 over the entire surface of the rotor 3. Therefore, the capacitance of the variable capacitor 1 is stabilized and varies smoothly with rotation of the rotor 3. Also, drift in the set position is stabilized, and torque required to rotate the rotor 3 becomes uniform.
When it is desired to make the variable capacitor 1 thinner, this can be effectively accomplished by making the rotor 3 thinner. However, when the rotor 3 is thinned to a thickness of 0.3 mm or less, a pressing force applied to the rotor 3 by the spring-action portion 15 may cause the rotor 3 to deform. Particularly, as in the case of the variable capacitor 1 shown in FIG. 7 in which the protrusions 16 are formed on the spring-action portion 15, the pressing force is applied to the rotor 3 in one or more localized regions. Thus, the rotor 3 is known to be susceptible to deformation. Such an undesirable deformation of the rotor 3 hinders smooth capacitance variation affected through rotation of the rotor 3, typically causing a problem in that the linearity of capacitance variation is impaired, and also rendering the set position (and set capacitance) subject to drift.